Measurement of skin protein breakdown in a rat model

Whereas skin protein synthesis can be measured with different approaches, n o method potentially applicable in humans is available for measurement of s kin protein breakdown. To that end, we measured mixed skin fractional prote in breakdown (FBR) in a rat model by use of a stable isotope method (tracee release method) originally developed to measure muscle protein breakdown. Skin mixed protein and collagen fractional synthesis rates (FSR) were also measured. A primed continuous infusion of L-[ring-H-2(5)]phenylalanine and alpha-[5,5,5-H-2(3)]ketoisocaproate (KIC) was given for 6 h. Arterial and s kin phenylalanine and leucine free enrichments were measured at plateau (5- 6 h) and during the decay that followed after the infusion was stopped. Ski n FBR (%/h) was 0.260 +/- 0.011 with phenylalanine and 0.201 +/- 0.032 with KIC/leucine [P = not significant (NS)]. Mixed skin FSR (%/h) was 0.169 +/- 0.055 with phenylalanine and 0.146 +/- 0.020 with KIC/leucine (P = NS). Co llagen FSR was 0.124 +/- 0.023%/h (P = NS vs. mixed protein FSR). The trace e release method is a sensitive method for measurement of skin protein brea kdown; however, given the high intersubject variability of FSR, the calcula tion of skin net balance is not advisable.